Although you might make a greater effort to attend your manager’s or an important client’s social affair than you would for that of a coworker or supplier, your decision should never be reflected in your response. Timing and correctness should always be the same.
The most important letters in the etiquette of invitations are RSVP. RSVP does not mean “Let us know if you’re coming.” It means, “Let us know either way.” Failure to reply to an invitation is a definite blunder (unless you’ve somehow contributed money for the invitation). Of course if the invitation specifies “Regrets only,” you needn’t acknowledge acceptance, but you do need to let the host know if you cannot make it.